The present invention is related to the field of printed circuit boards, and in particular to the manufacture of rigid/flex printed circuit boards having both rigid and flexible areas so as to be mountable in non-planar spaces.
Printed circuit board technology has evolved such that it is presently possible to fabricate so-called "rigid/flex" circuit boards that have desirable characteristics of both rigid and flexible circuits. Relatively rigid areas of rigid/flex boards are used for mounting circuit components, attaching external cables via connectors, and providing electrical circuit interconnections among the components and connectors. Relatively flexible areas of the rigid/flex boards are used to carry electrical interconnections and to enable the board to be bent or shaped as required by the physical characteristics of the system in which the board is to be used. For example, it is known to use a rigid/flex having two rigid areas connected by a flex area and folded into a "U" shape to attain higher volumetric component density than might be achieved using two separate rigid circuit boards and conventional interconnection technology.
Rigid/flex circuit boards have a variety of structures, and have been made by a variety of methods. Some rigid/flex circuit boards are fabricated by laminating a series of flexible layers together, alternating insulative and conductive layers as necessary. One technique for forming flexible areas involves the selective removal of portions of outer laminated layers. Where the outer layers have been removed, the board is relatively thin, and thus more flexible than in surrounding areas where the laminated outer layers have not been removed.
One method of fabricating rigid/flex boards employs a flexible insulative core that is copper-cladded on both sides. The copper layers are selectively etched to form circuit traces. Then an insulating layer and another conductive layer are laminated to each side, resulting in a rigid/flex board having four or more conductive layers. Prior to lamination, the outer insulating layers are provided with openings where flexible areas of the rigid/flex circuit board will exist. The openings may be formed for example by routing. The outer conductive layers are formed of thin, conductive, metallic foil such as copper foil. Because of their fragility, it is generally not feasible to etch or shape the outer conductive layers prior to lamination. Rather, the foil is first laminated to the circuit board for mechanical stability, and then portions of the laminated foil are removed by suitable means such as chemical etching.
One potential problem that arises in the above-described process is that the copper traces on the inner layers adjacent to the core are exposed in the areas where openings have been formed in the laminated outer insulating layers. Steps must be taken to protect these exposed traces prior to etching the outer laminated foil, so that the etchant does not attack the exposed metal traces. As a general matter, prior techniques have employed one or more additional process steps to deposit a "mask" of inert material on the flexible circuit prior to the lamination of the pre-processed insulating outer layers. The mask material is deposited or laminated in the areas where the openings in the outer insulating layers are formed. Subsequently, the edges of the mask pattern are sealed in some manner to the surrounding outer layers, such as for example by "tucking" the mask material under a subsequently-laminated layer. The mask material protects the circuit etch on the flexible circuit during the etching of the outer conductive layers.
There are drawbacks with the use of such masking techniques. From a reliability viewpoint, it is difficult to obtain a good seal between the mask material and the subsequently-applied outer insulating layers at the periphery of the window areas. Thus it is possible that etchant or other contaminants are introduced into the circuit structure, causing damage and shortening the lifetime of the circuit board. From a cost viewpoint, it is undesirable to require the use of a separate masking step prior to etching the outer conductive layers, because each additional step in fabrication increases the cost and complexity of the overall process and resulting rigid/flex board.
It would be desirable to improve the manufacturing of rigid/flex circuit boards to reduce costs and complexity, while maintaining or enhancing the reliability of the fabricated circuit boards.